Orthogonal frequency division multiplexing (OFDM) provides a useful way to modulate data for transmission. OFDM may be considered a form of digital multi-carrier modulation. A number of orthogonal sub-carriers are used to carry data. Data for transmission is then divided into several parallel data streams for transmission. Each of the sub-carriers may in turn be modulated using binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), quadrature amplitude modulation (QAM), and so forth.
An OFDM system uses several carriers, or “tones,” for functions including data, pilot, guard, and direct component. Data tones are used to transfer information between the transmitter and receiver via one of the channels. Pilot tones are used to maintain the channels, and may provide information about time/frequency and channel tracking. Guard tones may be inserted between symbols during transmission to avoid inter-symbol interference (ISI), such as might result from multi-path distortion. These guard tones also help the signal conform to a spectral mask. The nulling of the direct component or DC may be used to simplify direct conversion receiver designs.
In certain instances, an OFDM system, in order to send out OFDM symbols, implements padding bits. Bit padding allows a full OFDM symbol to be sent, although the padding bits per se are not needed. Bit padding typically may be implemented at the physical or PHY layer.
The Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard defines protocols for wireless transmission. As the IEEE 802.11 standard evolves, support for new tone allocations, modulation and coding can create issues and problems. In particular, a condition can arise, where the PHY layer bit padding used in legacy IEEE 802.11 systems may not work.